1. Field of the Invention
This invention relates to data collection and processing of that data in digital apparatus which performs metering functions and can also perform harmonic distortion analysis of the waveform in an ac electrical power system.
2. Background Information
State of the art monitors for ac power systems incorporate microcomputers for calculating various electrical parameters such as rms currents and voltages, peak currents and voltages, power, energy, power factor, and the like. Typically, the sampling rate at which the analog waveforms of the electrical power system are digitized for input to the microprocessor is a compromise between the high sampling rate desired for increased accuracy, and lower rates imposed by computing time needed for the processor to calculate the various electrical parameter desired as outputs.
Waveform analyzers are used for oscillographic analysis of the waveforms in the ac electrical power system and can also be used to determine the harmonic content of the waveform. In accordance with the well known Nyquist criteria, a signal must be sampled at twice the highest frequency to be detected. Thus, a waveform must be sampled at twice the frequency of the highest harmonic to be extracted. For example, a 60 Hz ac signal must be sampled at least at 6 KHz in order to extract the 50th harmonic. This high sampling rate places a burden on the microcomputer. In fact, in one monitor/analyzer, only the monitoring function such as calculation of the various voltages and currents, power and the like are performed in the microcomputer of the device. The raw digital waveform data is sent to a remote computer with greater computing capacity for performing the harmonic analysis.
State of the art circuit breakers also utilize microcomputers in the trip unit. Such digital trip units can perform monitoring functions in addition to the protection functions. Some of these circuit breakers employ what is known as an equivalent sampling technique to increase accuracy without placing an undue burden on the microprocessor. In the equivalent sampling technique, the ac waveforms are sampled a selected number of times per cycle with a delay of a fraction of a cycle before another cycle of samples is taken at the same sampling rate. Thus, the sampling instants are "bumped" each cycle by the selected fraction of a cycle. The data collected over a number of such "bumped" cycles are then used to calculate the various parameters. For instance, if a sampling rate of 16 samples per cycle is used, an effective rate of 64 samples per cycle can be realized by sampling for one cycle, delaying for 1/64 of a cycle and then taking another 16 samples at the rate of 16 samples per cycle. This is repeated until four cycles of data are accumulated; however, 4 1/16 cycles are required to generate this data. Thus, this is not synchronous sampling, but then synchronous sampling is not necessary to perform the monitoring and protection functions.
However, sampling must be synchronous in order to perform the Fourier analysis used for calculating harmonic distortion. By synchronous sampling, it is meant that an integer number of samples are taken per cycle. In addition, as mentioned above, a high sampling rate is required to detect the full range of harmonic information needed to make the harmonic analysis. At the same time, the Fourier analysis of that data requires a considerable amount of computation time. The result is that a very high demand is placed upon the microcomputer, especially if extensive monitoring is also to be performed by the device.
There is a need therefore, for a digital monitor/analyzer for ac power systems which can perform harmonic distortion analysis internally and also perform extensive monitoring functions.
In particular, there is a need for such an improved digital monitor/analyzer which can sample the ac waveforms at a high enough rate to obtain the data needed for a full harmonic distortion analysis and at the same time have enough computational time for performing that analysis while also performing extensive monitoring calculations.